1. Field of the Invention
This invention relates to amplifier circuits. In particular, the invention describes a system to incorporate an operational amplifier and a feedback loop on a single silicon chip.
2. Description of Related Art
Networks (including resistive, capacitive, inductive, and currents source networks) are commonly used in analog integrated circuits. For example, resistive networks are commonly used to provide desired reference voltage levels. Resistive elements are formed on integrated circuits and are thus subject to process variations inherent in integrated circuit manufacturing processes. Within a typical process, the resistance of a given resistive element may deviate from a desired resistance by plus or minus five percent. In some applications, such large deviations are unacceptable. To obtain tighter tolerances, prior art references "trim" resistive circuits by setting "links" to vary resistive parameters.
Trimming is typically performed by a trim circuit coupled to a resistive network. The trim circuit creates a trim signal which controls switches within the resistive network. An ohm meter measures the resistance of the resistive network while a control signal opens and closes switches to determine how much trimming is desirable to achieve a desired resistance.
Various methods of trimming the resistor network are described in prior art references and illustrated in FIGS. 1A and 1B. In one embodiment, a trimming is accomplished by directing a laser beam 104 at the "sandwich" structure 108 of a target resistor element 112. The laser beam 104 does not vaporize materials at the point of contact but causes a recrystallization and scattering of atoms which programs the target resistor element 112 into a high resistance state. In one embodiment, the changing of the resistance value is several orders of magnitude to simulate the operation of a switch.
Another method of trimming a resister utilizes an anti-fuse resistive element 150 which includes a bottom conductive layer 152, an oxide layer 154 provided with a hole 156, and an upper conductive layer 158. The hole may be filled with an anti-fuse material 160 such as amorphous silicon. Prior to programming, the anti-fuse resistive element 150 approximates an open circuit. The anti-fuse resistive element 150 may be programmed using a large voltage potential which causes conductive pathways to form through the anti-fuse material lowering the resistance of resistive element 150.
Other methods of trimming a resistive network are described in the prior art reference, Low Powertrim Circuit and Method, U.S. Pat. No. 5,563,549, by Sui Shieh.
Operational amplifiers are often connected to resistor networks to adjust the closed loop gain of an amplifier circuit. However, current methods of adjusting a resistor network and connecting the resistor network to the operational amplifier are inaccurate, and require additional packaging. In addition, varying resistive parameters to achieve a desired gain often results in a poor frequency response of the amplifier circuit. Thus, a more efficient method and apparatus for producing an op-amp circuit with a resistive network is needed.